Integral separator start-up system for a vapor generator with constant pressure furnace circuitry

ABSTRACT

A vapor generating system for use in a power plant having a steam turbine and a condenser connected in a series flow relationship, in which a vapor generating section receives a heat exchange fluid and passes the fluid in a heat exchange relation to a source of heat to raise the temperature and pressure of the fluid to predetermined values. The vapor generating section operates at substantially constant pressure during start-up and a pressure reducing station is provided for reducing the pressure of the fluid before it is passed to a plurality of separators in the main flow path for receiving the fluid and separating the fluid into a liquid and a vapor. The separators are connected to a vapor circuit and to a liquid circuit for passing the vapor and liquid to the turbine and condenser, respectively. A method of starting up a vapor generating system in a power plant.

BACKGROUND OF THE INVENTION

This invention relates to a start-up system for vapor generators and in particular to an apparatus and method for starting up a sub-critical or super-critical, once-through vapor generator.

In general, a once-through vapor generator operates to circulate a pressurized fluid, usually water, through a vapor generating section and a superheating section to convert the water to vapor. In these arrangements, the water entering the unit makes a single pass through the circuitry and discharges through the superheating section outlet of the unit as superheated vapor for use in driving a turbine, or the like.

Although these arrangements provide several improvements over conventional drum-type boilers, some problems have arisen in connection with starting up the generators, usually stemming from fluid at an undesirable quantity or condition being passed to the components of the system, resulting in excessive thermal losses, as well as mismatching of the throttle steam to the turbine inlet causing a decrease in turbine component life.

Earlier attempts to solve some of these problems included arrangements providing bypass circuitry for a portion of the fluid at a point in the flow circuitry between the vapor generating and superheating sections and/or between the superheating section and the turbine during start-up to precool a portion of the system yet avoid the possibility of fluid at an undesirable quantity or condition being passed to the turbine. However, these arrangements resulted in very poor heat recovery and, therefore, operated at a reduced thermal efficiency and, moreover, resulted in relatively unsuitable turbine throttle vapor conditions for rolling and bringing the turbine up to speed prior to loading.

Attempts to alleviate the latter problems included installing a division valve in the main flow path to divert flow to a bypass circuit including a flash tank separator located between the vapor generating section and the superheating section, or between a primary and finishing superheater in the superheating section. In these arrangements, the flash vapor from the separator is furnished to the superheating section or to the finishing superheater, and the drains from the separator are passed to a deaerator and/or high pressure heater. However, in these systems, the separator could often accommodate only a limited pressure, which was considerably less than the full operating pressure of the main pressure parts. Therefore, after start-up, when turbine demands approached pressures exceeding the design pressure of the separator, the separator had to be switched out of operation and flow to the turbine supplied directly from the main flow line upstream of the flash tank. However, this switch of flow often caused control difficulties and, in addition, caused a drop in enthalpy at the turbine since the flow source switched from a saturated vapor from the separator to a lower enthalpy water-vapor mixture from the main flow line. Therefore, in order to avoid pressure excursions and an uncontrolled significant temperature drop at the turbine throttle, the valve controlling flow to the turbine directly from the main flow line had to be opened very slowly, the firing rate had to be increased, and the separator outlet valve closed to slowly transfer the source of turbine steam from the separator to the main flow line. This, of course, resulted in a considerable expenditure of time and energy, and a considerable sophistication of controls.

Also, in these latter arrangements, when vapor formed in the separator in response to a start-up firing rate input, the vapor, in addition to flowing to the turbine, was routed to other areas of the system such as high pressure heaters and/or the condenser until a percentage of the final turbine load was achieved. Therefore, these arrangments required the use and operation of several valves which added to the labor and costs in the operation of the system.

Although it has been suggested to provide a separator directly in the main flow line, these arrangements have proven to be costly due to the fact that a relatively large, thick-walled separator, and associated components, have to be used. Also, the vapor initially forming in the separator is passed in a circuit bypassing the finishing superheater and the turbine during start-up, after which the flow is switched to the superheater and turbine, which also requires a control system utilizing a number of valves.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a vapor generating system and method incorporating a start-up system which does not require the use of bypass circuitry incorporating a flash tank separator.

It is a further object of the present invention to provide a system and method of the above type in which a plurality of separators are utilized which together operate in a manner to permit the turbine to be smoothly loaded at pressures and temperatures that constantly and gradually increase.

It is a still further object of the present invention to provide a system and method of the above type in which a pressure reducing station is located upstream of the separators to supply the fluid to the separators at a pressure permitting the liquid portion of the fluid to be separated from the vapor portion.

It is a still further object of the present invention to provide a system and method of the above type in which vapor initially forming in the separators is immediately passed in the main vapor circuit containing the superheater section and the turbine, to eliminate the controls and valves required to initially route the vapor elsewhere.

Toward the fulfillment of these and other objects, the system of the present invention comprises a vapor generating section adapted to receive a heat exchange fluid and pass the fluid in a heat exchange relation to a source of heat to raise the temperature of the fluid to predetermined values, the vapor generating section adapted to operate at constant pressure during start-up, pressure reducing means connected to the outlet of said vapor generating section for receiving the fluid and reducing the pressure thereof, a plurality of separators connected to said pressure reducing means and adapted to receive the fluid from the pressure reducing means, each separator adapted to separate the fluid into a liquid and a vapor; and vapor circuit means connecting said separators to the turbine for passing all of the vapor formed in the separators to the turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The above brief description, as well as further objects, features, and advantages, of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the present invention, when taken in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a power plant incorporating the system of the present invention;

FIG. 2 is a partial sectional, partial schematic view of the vapor generating section of the system of the present invention;

FIG. 3 is a schematic perspective view illustrating the furnace circuit used in the system of the present invention;

FIG. 4 is a vertical cross-sectional view of a separator used in the system of the present invention;

FIG. 5 depicts a flow circuit through and between several components of the system of the present invention;

FIGS. 6-10 are a series of schematic diagrams, similar to FIG. 1 and illustrating the various operational modes of the system of the present invention; and

FIG. 11 is a pair of operational curves derived in accordance with the system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, there is depicted the main fluid circuit of the system of the present invention, which includes an economizer 10 which receives a heat exchange fluid, such as water, and transfers same to a furnace section 12 which is adapted to convert the water to steam in a manner to be described. A pressure reducing station 14 is located downstream from the furnace section 12, and a separating station 16 is in direct flow connection with the outlet of the pressure reducing station. A primary superheater 18 is located downstream of the separating station 16 and, in turn, is connected to a finishing superheater 20. A high pressure turbine 22 receives fluid from the finishing superheater 20 after it passes through a stop valve 24, and the exhaust flow from the high pressure turbine 22 is reheated in a reheater 26 before being transmitted to a low pressure turbine 28. The economizer 10, the furnace section 12, the superheaters 18 and 20, and the reheater 26 are all part of a vapor generating section which will be described in detail later.

Also included in the main flow path is a condenser 30 which receives the exhaust flow from the low pressure turbine 28 and which includes a hotwell 32. Between the hotwell 32 and the economizer 10, the main flow path includes a demineralizer 34 which receives the condensate from the hotwell 32 under the force of a pump 36, a series of low pressure heaters, shown in general by the reference numeral 38, and a deaerator 40 which transmits the flow to a feedwater pump 42. The water is pressurized in the feedwater pump 42 and transmitted to the tube sides of two high pressure heaters 44 and 46, and from there back to the economizer 10 to complete the main flow circuit.

It is understood that the connections between the above-mentioned sections in the main flow circuit which are shown schematically in the drawings are achieved by fluid circuitry in the form of conduits, tubes, risers, headers, etc., to transfer the heat exchange fluid throughout the various sections.

Other components associated with the main flow circuit include a spray station 48 located in the circuit between the primary superheater 18 and the finishing superheater 20, and a turbine stop valve bypass 50 connected in parallel with the turbine stop valve 24 between the finishing superheater 20 and the high pressure turbine 22.

The separating station 16 includes a plurality of separators which will be described in detail later and which operate to receive a mixture of vapor and water and separate the vapor from the water. In this context, a water flow circuit, including a drain line 52, connects the water outlets of each of the separators to a drain collecting header 54 from which the water passes through an isolating valve 56 and to control valves 58 and 60 arranged in parallel. A level control unit 62 is responsive to level changes in the separators in a manner to be described, for controlling the operation of the control valves 58 and 60 to maintain predetermined water levels in the separators. Valves 64, 66 and 68 are disposed in the above-mentioned water flow circuit to selectively direct the water from the control valves 58 and 60 to the condenser 30, the deaerator 40, and to the high pressure heater 46, respectively.

Other ancillary fluid flow circuits include lines 70a and 70b connecting the output of the high pressure heater 46 to the hotwell 32 and the deaerator 40, respectively, under the control of valves 72a and 72b, respectively; and a line 74 connecting the generator side of the stop valve 24 to the condenser 30 under the control of a valve 76 for the purposes of providing a drain for the main fluid circuit to the condenser 30.

Reference is now made to FIG. 2 which depicts the above-mentioned vapor generating section of the present invention which includes several of the above-mentioned components in greater detail. In particular, the vapor generating section is referred to in general by the reference numeral 80 and includes the furnace section 12 which is formed by front and rear walls 82 and 84, respectively, and a pair of sidewalls 88 (one of which is shown in FIG. 2) which extend between the front and rear walls to define an enclosure. The lower portions of the front wall 82 and the rear wall 84 are sloped inwardly to form a hopper section 92. It is understood that each of the walls 82, 84, and 88 are made up of a plurality of vertically extending tubes having continuous fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form an airtight structure.

The front wall 82 and both sidewalls 88 form continuous panels extending vertically from the hopper portion 92 to a roof 94 which is also formed of a plurality of tubes arranged in the above manner. The upper portion of the rear wall 84 is configured in a manner to be described later so as to define a plurality of openings which, together with the roof 94, define a gas exit 96 which leads to a vestibule 98 in gas flow communication with a downwardly extending convection zone 100 having an outlet 102. The vestibule 98 and the convection zone 100 house the economizer 10, the primary superheater 18, the finishing superheater 20, and the reheater 26, all of which are in the form of a plurality of tubes adapted to receive the heat exchange fluid.

A plurality of burners 104 and 106 are disposed in the front and rear walls 82 and 84, respectively, immediately above the hopper section 92, with the burners being arranged in several vertical rows of four burners per row.

As a result of the above, the flow of gases in the vapor generating section is upwardly through the furnace section 12 and through the gas exit 96 into the vestibule 98 and downwardly in the convection zone 100 to the outlet 102. It is understood that an air heater (not shown) is connected to the outlet 102 for effecting a heat exchange between the hot exhaust gases and incoming air for the burners in a conventional manner.

The details of the particular arrangement of the furnace section 12 are better depicted in FIG. 3. In particular, the upper portion of the rear wall 84 has a branch portion 84a which is formed by bending a selected number of tubes from the upper portion of the rear wall 84 to provide adequate spacing between some of the tubes in both of the walls 84 and 84a to define the exit 96 and permit the gases to pass from the vestibule portion 98 into the convection zone 100. A plurality of spaced parallel division walls 110 extend within the upper portion of the furnace enclosure with their lower portions being bent to penetrate the front wall 82 at a location above the burners 104, with the penetrating end portions of the division walls being connected to an inlet header 112.

The fluid flow circuit through the vapor generating section will be described in connection with FIGS. 2 and 3. In particular, feedwater from an external source (not shown) is passed to the tubes of the economizer 10 where it is heated by the gases passing over the latter tubes before exiting from the outlet 102. The heated feedwater from the economizer 10 is then passed via a line 113 (FIG. 2) to the inlet header 112 of the division walls 110 where it flows upwardly through the tubes of the walls to a plurality of outlet headers 114, and, via a header 116 and a downcomer 118, to an inlet header 120 which is in registry with the lower ends of the tubes of the front wall 82, as shown in FIGS. 2 and 3. An outlet header 122 is in registry with the upper ends of the tubes of the wall 82 for receiving the water after it passes through the latter wall. A downcomer 124 connects the outlet header 122 to inlet headers 126 which register with the lower ends of the tubes of both sidewalls 88 with the downcomer being branched to provide equal flow to both inlet headers. The fluid flow is thus transmitted upwardly through both sidewalls 88 where it is collected in headers 128 registering with the upper ends of the tubes of the latter walls. The fluid is then transferred, via a header 130 and a downcomer 132 to an inlet header 134 registering with the lower ends of the tubes of the rear wall 84. The fluid then flows upwardly through the tubes of both the rear wall 84 and the branch wall 84a in a parallel fashion before it is respectively passed, via outlet headers 136 and 136a, respectively, to an inlet header 138 connected to the tubes forming the roof 94, for further treatment that will be described later.

As a result of the foregoing, the water flows upwardly through the walls of the furnace 12 in essentially four vertically oriented flow passes connected in series -- a first pass through the division walls 110, a second pass through the front wall 82, a third pass through the sidewalls 88, and a fourth pass through the rear wall 84. As a result of this type of multipass flow, the furnace operates at constant pressure during start-up, as will be described in detail later.

FIG. 4 depicts, in detail, one of the separators used in the separating station 16 in the main flow circuit. The separator is referred to in general by the reference numeral 140 and includes an upright cylindrical shell 142 through which a riser pipe 144 extends in a coaxial relationship. The riser pipe 144 has an end portion 144a extending from the lower end of the shell 141 which is adapted for receiving fluid from the pressure reducing station 14. A cap 146 extends over the upper end of the pipe 114 and a plurality of slots 148 are formed through the upper wall portion of the pipe near the latter end. A plurality of substantially spiral-shaped arms 150 are connected to the pipe 144 in registry with the slots 148 with the free ends of the arms being open to permit fluid to discharge therefrom.

A cylindrical, open-ended, skirt 152 extends within the shell 142 and around the upper portion of the riser pipe 144 in a coaxial relationship therewith, with the inner wall of the skirt being spaced a small distance from the free ends of the arms 150. The skirt 152 is supported relative to the shell 142 in the position shown by spacers 151 and a plurality of set screws 153 extending through the shell and engaging the skirt.

A drip ring 154 is disposed in the upper portion of the shell 142 above the arms 150, and a cup-shaped thermal sleeve 156 extends over the lower end portion 144a of the riser pipe 144 to define an annular passage in communication with an auxiliary drain 158.

A vapor outlet nozzle 160 is provided at the upper end portion of the shell 142 and a radially extending drain water outlet 162 is provided near the lower end portion of the shell. Also, the shell 142 is provided with a high level connection 164 and a low level connection 166 disposed near the upper and lower end portions of the shell, respectively, for the purpose of maintaining predetermined water levels in the shell 142, as will be described later.

As a result of the above, when the fluid entering the end portion 144a of the riser pipe 144 from the pressure reducing station 14 is in the form of a vapor-water mixture, it passes upwardly in the riser pipe and then radially outwardly from the pipe through the slots 148 and into the arms 150 where it will be directed tangentially against the inner wall of the skirt 152. This creates a vortex, or whirling stream, of fluid with the resulting centrifugal forces causing the vapor portion of the fluid to travel away from the inner wall of the skirt 152 and towards the center of the whirling stream, and pass upwardly by virtue of its buoyancy into the upper portion of the shell 142, from which it exits, via the outlet 160. The liquid, or water, portion of the mixture in the whirling stream collects on and flows down the inner wall of the skirt 152 until it falls off the wall, collects in the lower end of the shell 142 and drains from the connection 162 for passage to and through line 52 of the fluid circuit, as previously mentioned in connection with FIG. 1.

As also shown in FIG. 1, the pressure reducing station 14 includes valves 170, 172, connected in parallel with valve 174 and capillary tubes 176 which are provided in the line associated with the valve 174, to selectively reduce the fluid pressure to the separating station 16 during initial start-up. The location and arrangement of the pressure reducing station 16, as well as the separators 140, are shown in FIGS. 2 and 5. Specifically, an outlet header 180 is in flow communication with the tubes forming the roof 94 and has one end registering with a downcomer 182, in which is disposed the valve 170 of the pressure reducing station. A bypass conduit 184 (FIG. 5) connects the header 180 with the downcomer 182, with the valve 172 being disposed in the conduit 184. The capillary tube 176, having the valve 174 disposed therein, connects the header 180 with the conduit 184 to form a third parallel connection between the header 180 and the downcomer 182 with the equivalent parallel connections being shown in FIGS. 1 and 2 for the convenience of presentation. The valve 174 and the capillary tubes 176 function to break down the pressure of the cold fluid during initial start-up, and thereby avoid excessive wear of the valve 172.

A plurality of conduits 190 connect the downcomer 182 to the end portions 144a of the risers 144 of a plurality of separators 140. It is understood that several of the separators 140 have been omitted from the drawings in the interest of brevity. Although also not shown in the drawings, it is also understood that another flow circuit identical to the one just described, connects the other end of the header 180 to another series of separators 140.

The vapor outlets 170 of the separators 140 are connected to an inlet header 192 for the primary superheater 18 as shown in FIG. 2 for supplying the vapor from the separators to the superheater. As previously discussed, the water outlets 162 of the separators 140 are connected, via a line 52 to the drain collecting header 54.

The basic operation of the system of the present invention is better described with reference to FIGS. 6-10 which are similar to FIG. 1 but show by means of relatively heavy lines and shading of the various components, the actual flow paths through the circuitry of the system during various operational stages. In particular, and with reference to FIG. 6, the system is initially flushed by actuating the feedwater pump 42 and opening the valves 174 to establish a feedwater pumping rate of approximately 15% of full load which routes the water through the economizer 10, the walls of the furnace section 12, the valves 174 and the capillary tubes 176 to maintain the upstream furnace circuit pressure at a range of 1800-2200 psi. The valves 66 and 68 are closed and the valve 64 opened so that the water draining from the separators 140 passes through the line 52, the header 54, and the valves 56, 58, and 60, to the condenser 30, via the valve 64. In this manner, cycle water clean-up can be accomplished by routing the flow through the demineralizer 34. Since no vapor is generated at this stage, there is no flow through the vapor flow circuit extending from the separators 140.

Control valves 58 and 60 will maintain normal water level in separators 140 in response to level control unit 62.

With reference to FIG. 7, the burners 104 and 106 are lit at approximately 10-15% of their full load firing rate and the temperature of the fluid entering the pressure reducing station 14 is raised over a period of time to approximately 450°. The feedwater pumping rate is increased to 25% which causes the valve 172 to open to control the outlet pressure from the furnace 12 to approximately 3500 psi. The valve 64 is closed and the valves 66 and 68 are opened so that drain water from the separators 140 is routed through the line 52, the header 54, and to the deaerator 40 and the high pressure heater 46 under control of the valves 66 and 68, respectively. The steam separated in the separators 140 as a result of the above-mentioned increased temperature is routed directly to and through the superheaters 18 and 20 and to the turbine stop valve 24 where it drains, through the line 74, into the condenser 30.

Referring to FIG. 8, the throttle pressure is then raised to 1000 psi. by increasing the firing rate of the burners 104 and 106, the turbine bypass valve 50 is opened to direct steam into the high pressure turbine 22, to bring the turbine up to speed and to synchronize and initially load the turbine. The load is then increased to 7% of full load at the 1000 psi. throttle pressure. Spray station 48 controls final steam temperature to turbine 22. In this mode, the water flow from the separators 140 is identical to that discussed in the previous mode.

The turbine load is then increased to 25% of full load with the turbine control valves approximately 25% open by increasing throttle pressure from 1000 psi. to 3500 psi. by firing rate control to achieve the operation shown in FIG. 9. In this mode, the turbine bypass valve 50 is shut off and the steam flow is through the turbine stop valve 24.

As separator pressure approaches 3000 psi. a separator pressure error override modifies the level control unit 62 signal to control valves 58 and 60 when low level is indicated. In this manner proper operation of valves 58 and 60 is assured even as the level control signal becomes erratic in the 3000 psi. to 3550 psi. range.

As the load approaches 25%, the larger control valve 58 closes with later sequential closing of the small control valve 60. At 25% load both valves 58 and 60 are closed removing the bypass water drain circuit, including line 52 from service, as shown in FIG. 10. As the turbine load is increased above 25%, the valve 170 at the pressure reducing station 14 is opened and vapor at full pressure from the furnace section 12 passes through separators 140 and routes to superheaters 18 and 20 and to the high pressure turbine 22, with the spray station 48 in service for transient control of final steam temperature.

It is thus noted that when the capillary tubes 176 are in service, furnace pressure will vary between approximately 1800 psi. and 2200 psi. with the valve 170 and 172 fully closed, and that when the fluid flow is increased from 15% to 25% of full load flow as described above, the valve 172 will open to control upstream furnace pressure to 3550 psi. Also, the stop valve 170 will open when 25% turbine load is achieved at 3550 psi. throttle pressure.

FIG. 11 depicts the variations of furnace pressure and the throttle pressure to the superheaters and high pressure turbines throughout the above-mentioned stages of operation and summarizes the pressure variances discussed in connection with FIGS. 6-10.

It is thus seen that the system and method of the present invention enables a quick and efficient start-up to be achieved with a minimum of control functions. Also, since the level in the separators 140 in the arrangement of the present invention determines the division point between water and steam circuits during start-up, the system and method of the present invention enables the turbines to be smoothly loaded at optimum pressures and temperatures that can be constantly and gradually increased, without the need of boiler division valves or external bypass circuitry for steam dumping. Also, according to the system and method of the present invention, operation can be continuous at very low loads with a minimum of heat loss to the condenser. Still further, it can be appreciated that the system and method of the present invention can operate in a cyclic mode, such as, for example, reduction to house load daily and/or weekly shut-downs and restarts.

Of course, it is understood that variations of the specific construction and arrangement of the system and method disclosed above can be made by those skilled in the art without departing from the invention as defined in the appended claims. 

What is claimed is:
 1. A vapor generating system for use in a power plant having a steam turbine and a condenser connected in a series flow relationship, said system comprising a vapor generating section for receiving a heat exchange fluid and applying heat to said fluid, a superheating section for applying additional heat to said fluid, fluid flow circuitry directly connecting said vapor generating section to said superheating section, pressure reducing means connected directly in said fluid flow circuitry between said vapor generating section and said superheating section for receiving said heated fluid from said vapor generating section and reducing the pressure thereof, separation means connected directly in said fluid flow circuitry between said pressure reducing means and said superheating section for receiving said heated fluid at said reduced pressure and separating said fluid into a liquid and a vapor, said fluid flow circuitry passing all of the vapor from said separation means to said superheating section during start-up and full load conditions.
 2. The system of claim 1, wherein said vapor generating section is adapted to operate at a substantially constant pressure during startup.
 3. The system of claim 1, wherein said superheating section comprises a primary superheater and finishing superheater connected in a series flow relationship.
 4. The system of claim 1, wherein said separation means comprises a plurality of separators connected in a parallel relationship, and further comprising means for controlling the liquid level in each of said separators.
 5. The system of claim 1, further comprising means connecting said separation means to said condenser for passing liquid formed in said separation means to said condenser.
 6. The system of claim 1, further comprising liquid circuit means connecting said condenser to said vapor generating section, and at least one heating means and at least one deaerator connected in said liquid circuit means in a series flow relationship between said condenser and said vapor generating section, the condensed vapor from said condenser being passed in said liquid circuit means through said heating means and said deaerator and to said vapor generating section.
 7. The system of claim 6, further comprising means for passing said liquid from said separation means directly to said heating means.
 8. The system of claim 6, further comprising means for passing said liquid from said separation means directly to said deaerator.
 9. The system of claim 1, further comprising means for connecting said superheating section to said turbine for passing said heated fluid to said turbine.
 10. The system of claim 1, wherein said pressure reducing means comprises at least one capillary tube, and valve means disposed in said tube.
 11. The system of claim 10, wherein said pressure reducing means comprises at least one flow conduit disposed in a parallel relationship to said capillary tube, and valve means disposed in said flow conduit.
 12. A method of generating vapor for use in a power plant having a steam turbine and a condenser connected in a series flow relationship, comprising the steps of passing said fluid through a vapor generating section to apply heat to said fluid, directly passing said fluid from said vapor generating section to a superheating section to apply additional heat to said fluid, reducing the pressure of said heated fluid as it passes from said vapor generating section to said superheating section, separating said fluid into a liquid and a vapor after said step of reducing and before the fluid is passed to said superheating section, all of the separated vapor being passed to said superheating section during start-up and full load conditions.
 13. The method of claim 12, further comprising the step of operating said vapor generating section at a substantially constant pressure during startup.
 14. The method of claim 12, further comprising the step of passing the separated liquid to said condenser.
 15. The method of claim 12, further comprising the step of connecting said condenser to said vapor generating section for passing the condensed vapor from said condenser to said vapor generating section.
 16. The method of claim 15, further comprising the step of heating and deaerating said condensed vapor as it passes from said condenser to said vapor generating section.
 17. The method of claim 12, further comprising the step of connecting said superheating section to said turbine for passing said heated fluid to said turbine. 